1. Field of the Invention
The present invention is concerned with the field of digital encoding of speech, audio and other signals based on analysis-by-synthesis techniques including, in particular but not exclusively, Multipulses, Code Excited Linear Prediction (CELP) and Algebraic-Code Excited Linear Prediction (ACELP). More specifically, the present invention relates to the eradication of an occasional instability found in these analysis-by-synthesis techniques.
2. Brief Description of the Prior Art
Analysis-by-synthesis techniques such as Multipulses, Code Excited Linear Prediction (CELP) and Algebraic-Code Excited Linear Prediction (ACELP) are subjected to occasional instability in particular in the occurrence of channel errors during the transmission of highly periodic signals such as high-frequency sine waves. To circumvent the problem, "Instability-eradication methods", also referred to as "Instability-protection methods", have been developed.
Analysis-by-synthesis speech encoding techniques operate on a frame by frame basis and rely on a speech production model involving the production of (i) a spectrum described by a set of spectral coefficients such as the Line Spectral Pairs (LSP), (ii) a description of an innovation signal typically by way of a codebook and code gain, (iii) a pitch lag, and (iv) its corresponding pitch gain.
At the decoder, a periodic excitation signal is applied to a synthesis filter to produce the output speech. The needed periodic excitation is constructed by adding the received innovation signal to a version of the past excitation signal, namely, reusing the excitation signal a pitch-lag ago multiplied by the pitch gain. Clearly, this construction method is recursive and therefore exhibits a propensity to instability if the pitch gain is allowed to exceed unity.
In analysis-by-synthesis speech encoding techniques, best results are obtained when the pitch gain is allowed to range up to values above one and typically up to 1.2. There is no intrinsic problem with using such a range insofar as the decoder follows rigorously the transmitted instructions from the encoder. However, the combination of channel error and high pitch gain values can bring about instabilities. These problems surfaced during the extensive test programs used by the International Telecommunication Union (ITU) and other standardization bodies.
In the ITU G.729 speech-coding recommendation the problem was solved using a method to anticipate at the encoder a problem potential by monitoring the past excitation.